Battery state-of-charge indicators are employed to give the user a measure of remaining charge in a battery. Such indicators can operate in different ways, by measuring different parameters which change over the life of the battery. For example, as a battery is discharged its voltage output will decrease, so that a device that monitors voltage can be used to gauge the remaining charge once this effect has been calibrated. As the battery is discharged the maximum current that can be discharged will also decrease, allowing a measure of discharged current to be used to estimate remaining battery life. In addition, by accurately measuring the amount of current which has been provided by the battery, and relating this to a known amount of total charge in the battery, an exact measure of remaining charge can be obtained.
External testers based on careful monitoring of voltage, current, and remaining charge using electronic means are typically accurate and reproducible. There are different examples of such devices, two examples of which can be found in U.S. Pat. No. 4,163,186 and U.S. Patent Application Publication No. 2006/0097699. While such testers can be quite accurate, they are seldom used by the public due to the necessity of buying and using a separate device, and the inconvenience of needing it on-hand whenever the remaining life of a battery needs to be estimated.
On-battery testers, while typically less accurate, are typically far more convenient since they are already integrated onto the surface of a battery when purchased by a consumer. In an embodiment of such device, the user is prompted to press one or two spots on the battery surface, resulting in the appearance of a “fuel gauge” type bar, whose height indicates remaining charge (for example, as disclosed in U.S. Pat. Nos. 4,723,656, 5,250,905, 5,339,024, 5,396,177, 5,607,790, 5,612,151, 5,614,333). Devices as marketed by Duracell, for example, rely on a thermochromic layer on top of a resistive conductor: When the spots are pressed by the user, current will flow through the resistive conductor, causing an increase in heat and a color change in the thermochromic ink. As the charge in the battery is depleted, the available current will decrease, causing a decrease in the amount of “activated” thermochromic ink. While convenient, these testers may not be very accurate due to the dependence of applied pressure on gauge reading, and each time the tester is used the battery is effectively being short-circuited which may decrease the life of the battery.
Other possible on-battery tester types include electrochromic coatings (for example, as disclosed in U.S. Pat. Nos. 5,849,046 and 5,667,538), but these have not so far been implemented in commercial devices.
Photonic crystals are materials having a periodic modulation in their refractive index. This modulation causes coherent diffraction effects which manifest themselves, for example, as reflections of particular bands of wavelengths from the photonic crystal, given the photonic crystal certain optical characteristics, such as a reflectance spectrum. The optical characteristics, such as the wavelengths of the reflectance spectrum, are directly related to the length scale of the periodic modulation. Therefore, by changing the periodicity of the photonic crystal structure, the optical characteristics of the photonic crystal are also changed.
Photonic crystal materials may be designed to be responsive to external stimuli, in particular electrical stimuli. An example of such a material is described in U.S. Pat. No. 7,364,673 and in U.S. patent application Ser. No. 11/831,679 the entirety of which is hereby incorporated by reference.
It may be desirable to provide an electrical property indicator suitable for on-battery use.